Quantitation of serum prostacyclin-binding in thrombotic thrombocytopenic purpura

We quantitated serum PGI2 binding of 8 normal subjects and two TTP (thrombotic thrombocytopenic purpura) patients by gel filtration and gel partition methods using a stable PGI2 analogue, iloprost. The dissociation constant (KD) and the binding capacity (or binding stoichiometry) determined for the normals were 94 +/- S.D. 19 microM and 1.8 +/- S.D. 0.5 mM (or 2.0 +/- .6, iloprost:HSA). Corresponding values for serum samples obtained from TTP patient I were KD 200 microM, and Bmax 2.3 mM in the acute phase, and 75 microM and 1.8 mM respectively in the remission phase. The serum samples from TTP patient II exhibited a higher KD. Values of 299 microM (acute phase) and 147 microM (remission phase) were obtained. The corresponding binding capacities were 2.1 mM and 1.5 mM. Binding affinity change appears to be the main factor which resulted in the PGI2 binding defect in TTP.